Influence of residual surface investment material on crystallization shrinkage of a mica-based glass-ceramic.

OBJECTIVE: It has been reported that residual surface investment material induced precipitation of a second phase in the surface of a mica-based castable glass-ceramic. The objective of this study was to investigate the influence of this precipitate on the crystallization shrinkage of the glass-ceramic. METHODS: In experiment 1, six glass bars were cast. The whole surface of three of the six bars was ground to remove residual surface investment material. Then, the six bars were treated thermally to induce crystallization. Dimensional change in the bars during the treatment was recorded. In experiment 2, 14 glass specimens were assigned to two groups. Seven specimens in the one group would form the second phase. The length of each specimen was measured pre- and post-crystallization, and the crystallization shrinkage value calculated. RESULTS: In experiment 1, all specimens showed shrinkage during thermal processing. The majority of crystallization shrinkage began around 620 degrees C and finished at 750 degrees C. Specimens with the residual investment showed less net shrinkage than those without the investment. In experiment 2, mean shrinkage values and standard deviations were 1.11 (0.05)% for the group without the investment and 0.93 (0.11)% for the group with the investment. There was a significant difference between the groups (P<0.01). SIGNIFICANCE: The residual surface investment material decreases the crystallization shrinkage, and must affect the strength of the mica-based glass-ceramic and the fit of this ceramic restorative to the prepared cavity.